I've tried both DOM Inspector and the built-in Inspect in 4.0b5pre and both
show do not show the internal details of menu items in the Add-ons Manager.
The farthest the DOM tree opens is:
richlistbox#addon-list.list
-xul:scrollbox
--xul:box.box-inherit.scrollbox-innerbox
In the previous build I could inspect anything in the list items.